I received the same letter too, much to my delight I must say as i've been waiting 14 months and heard nothing back.
In my reply confirmation I asked politely for a breakdown of what to expect, seeing no harm in trying. I don't expect a response though as there's little point in DHL making it easy!
What I can suggest is looking a DHL's general recruitment website to find out exactly what kind of organisation it is. Scouring Flight International magazine for contempory industry issues would also be a plus.
On the day im planning just to turn up smart, on time and be myself (as far as appropriate). From what i've read phase 1 interviews are, in general, just to see that you can work as a team player, can fly an aircraft ok, can cope with a little stress and are as good as you looked on paper. To fail having got an ATPL you would have to be majorly deficient in some area. Its the phase 2, the personal interviews where they really screen the men from the boys. But thats a bridge to cross once part one is done.